Abrasion resistant steel, needless to say, is required to have abrasion resistance property stable over a long term and to be able to withstand long term use. For the various types of damage given from the environment during use of abrasion resistant steel, previous inventions have disclosed improvement in the delayed cracking resistance and hot cracking resistance and furthermore the low temperature toughness envisioning use at low temperatures etc.
For example, as art providing technology for production of steel plate superior in delayed fracture resistance by means of reducing the Mn (for example, see Japanese Patent Publication (A) No. 60-59019) and furthermore the art of applying a method of treatment tempering the steel at a low temperature of 200 to 500° C. (for example, Japanese Patent Publication (A) No. 63-317623) have been reported.
For the purpose of providing steel superior in hot cracking resistance, the technology of production limiting the Mn, Cr, Mo, and other ingredients (for example, see Japanese Patent Publication (A) No. 1-172514) and, furthermore, as technology for production of steel superior in low temperature toughness, the technology of mainly using alloy elements and limiting these ingredients (see, for example, Japanese Patent Publication (A) No. 2001-49387, Japanese Patent Publication (A) No. 2005-179783, and Japanese Patent Publication (A) No. 2004-10996) have been disclosed.
The above inventions are superior inventions in line with their objectives, but no invention can be found at present able to maintain a hardness stable over a long period of time, the most basic property expected from general abrasion resistant steel, that is, taking note of the change of hardness of a material used for a long period at close to room temperature.